1. Field of the Invention
This invention relates to an apparatus and a method to analyse the surface content of a solid state sample by mass spectrometric means.
Often it is necessary to analyse the content of a surface of some solid state sample. This can be done by bringing the substances contained by the surface into the gasphase preferably by evaporating some of the material off the surface. Some of this material must then be entered into a mass spectrometer and be ionized there. Some methods of analysis combine the two steps of evaporation and ionization into a single step, some other methods perform these steps separately.
In a time-of-flight mass-spectrometer the extraction volume is that region within the ion source of the mass-spectrometer, from which, upon start-time, ion paths lead to the surface of the detector of the time-off-light mass-spectrometer. The paths of the ions are given by the electrical fields and the physical laws of motion within.
The start-time of time-of-flight analysis can be given by:
the point of time, when neutral particles of a gas are ionized within the extraction volume by a laser or electron beam crossing it. PA1 the point of time when the electrode voltages of the ion source are switched on. This is usually the case when ions are to be analysed, since ions can only reach the extraction volume, when the voltages on the electrodes of the ion source are switched off. PA1 a) This means that so much substance is evaporated with just one shot of the laser, that it would saturate the mass spectrometer by orders of magnitude, if all of that substance would actually enter the mass spectrometer. PA1 b) Since each shot of the evaporating laser carries off these large amounts of material, the mass spectrometer shows a good signal even with losses on the transport path amounting to several orders of magnitute. Vice versa, if it is possible to reduce the amount of sample evaporated with one laser shot, then it also makes sense to reduce the losses on the transport path into the ion optics of the mass spectrometer. PA1 c) Since one laser shot evaporates such large amounts of material from the thin-layer chromatographic plate, this process cannot take place in the vacuum chamber of the ion optics of the mass spectrometer without the gas pressure there rising to unacceptable values.
Stated generally, the extraction volume is that region of space within the mass spectrometer, where ions have to be existant or must be produced, if they should give some signal on the detector of the mass spectrometer upon mass analysis.
2. Description of Related Art
State of the art shows a number of methods for bringing substances contained in thin surface layers into the gas phase and ionizing them. An example of this method is MALDI (Matrix Assisted Laser Desorption Ionization). Here the analyte substance is mixed into a "matrix", e.g. nicotinic acid, and coated onto a surface. After drying this sample can be introduced into a mass spectrometer. The principle of this method is that the matrix strongly absorbs at the wavelength of the laser desorbing this material. When a laser pulse strikes the surface a small portion of it will explode into the mass spectrometer. Since the analyte molecules are contained within the exploding matrix they also will be swept into the mass spectrometer. The analyte molecules can be ionized by this process of desorption or it is also possible that they are already present in ionized state within the matrix layer. In either case they are detectable by this process in a mass spectrometer. An overview of this method can be found in the publication of B. Spengler et al. (Analusis, vol. 20, pages 91-101, 1992).
Often desorption from the surface and ionization of the analyte molecules are two distinct processes. Specifically this is the case when the analyte substance are only present in neutral state in the surface material or cannot be ionized during the process of desorption with sufficient probablity. M. S. de Vries et al. (Review of Scientific Instruments, vol. 63, pages 3321-3325, 1992) use an UV-laser for desorption which is focused onto a spot of micrometer diameter by a microscope objective. The substances desorbed from this spot are ionized by a further laser and detected in a time-of-flight mass spectrometer. P. Voumard et al. (Review of Scientific Instruments, vol. 64, pages 2215-2220, 1993) also use two lasers, one for the desorption, the other for detection. However they use as desorbing laser instead an infrared laser.
Another way of separating the processes of desorption and ionization is to use one laser to bring the analyte substances in the gas phase, then using a pulsed gas beam to transport the analyte substance to some other location, ionizing them at that other location with a second laser pulse. This method can be advantageous when large amounts of unwanted material are also produced in the process of vaporization. This method can also be used to cool the analyte substances, when their detection and spectroscopic analysis is done by resonant multiphoton ionization. This variant of the method can be used for the analysis of thin-layer chromatographic plates.
Thin-layer chromatographic plates usually have dimensions in the order of 10 cm.times.10 cm. They are made of some inert base material onto which as transport layer e.g. a layer of silica gel is coated.
A mixture of analyte substances is coated near the edge of the plate in a start zone. After drying of the start zone the thin-layer chromatographic plate is inserted with the edge of the start zone into some suitable solvent. By means of capillary forces the solvent starts to climb up the plate taking the analyte substances along. According to their varying adsorption coefficients on the material of the transport layer the analyte substances follow the solvent motion with different velocities.
After some time the plate is removed from the solvent reservoir and dried. The dissolved substances will remain at their current location on the plate. Since these substances had different transport velocities on the transport layer they now will be on different locations on the plate with respect to the edge that was held into the solvent. It is of great interest to analyse these substances by mass spectrometric means.
T. Fanibanda et al. (International Journal of Mass Spectrometry and Ion Processes, vol. 140, pages 127-132, 1994) use a pulsed infrared laser to evaporate some silica gel together with the analyte substances from a slice of a thin-layer chromatographic plate. At the instance when the evaporated material is closely above the plate, it is swept by a pulsed CO.sub.2 gas beam through a scimmer into a time-of-flight mass-spectrometer. When the analyte substances reach the ion optics of this mass spectrometer a second laser is fired to ionize the analyte substances. The ionizing laser operates at a wavelength of 266 nm, which is useful for ionizing a great variety of molecules via multiphoton ionization.
Likewise, A. N. Krutchinsky et al. (Journal of Mass Spectrometry, vol. 30, pages 375-379, 1995) also use a pulsed infrared laser to evaporate material from a slice of a thin-layer chromatographic plate. Just as T. Fanibanda et al. they transport the analyte substances with a pulsed gas beam into the ion optics of their time-of-flight mass-spectrometer. However, contrary to T. Fanibanda et al. they use a tunable UV-laser for ionization which allows a higher selectivity when ionizing the analyte molecules with resonant multiphoton ionization via intermediate electronic states.
A somewhat more general method of analysing substances contained in the surface of solid state samples is presented in the UK patent application GB 2 149569 A. In this method a deflecting mirror is positioned directly above the surface of the sample. The laser beam strikes this mirror in a direction essentially parallel to the surface of the sample and is then deflected onto the surface of the sample. Between mirror and sample a lens is positioned which focuses the radiation onto a small spot on the surface. Both elements, the lens and the mirror have openings through which desorbed substances can reach an analysing instrument e.g. a mass spectrometer.
The above examples have shown some of a multitude of methods for analysing substances contained in surfaces of solid state samples. However, all of these examples have drawbacks. The experimental arrangements of B. Spengler et al., M. S. de Vries et al., and P. Voumard et al. have the disadvantage that the analysed surfaces can influence the electrical field in the ion source of their time-of-flight mass-spectrometer. This makes the construction of the ion source and the sample holder more complicated, also restricting the size of the analysed samples. The surfaces on the sample holder can vary from one experiment to another, as a consequence modifying the electrical field in the ion source of the mass spectrometer. Modification of the electrical field can deteriorate the mass resolution and/or sensitivity of the instrument.
Using the arrangements of T. Fanibanda et al. and A. N. Krutchinsky et al. the thin-layer chromatographic plates must first be cut into small slices before analysis in the mass spectrometer. This must be done in order to come with the sample surface as close as possible to the gas beam, and also not to unduly influence the expanding gas beam. Even then only some small part of the evaporated material is actually taken along by the gas beam, causing a great reduction in sensitivity. Then, the distance from the pulsed gas nozzle into the ion optics of the mass-spectrometer is quite long, meaning that only a small solid angle of the total expansion will pass through the interaction zone of the ionizing laser beam.
As another disadvantage the silica gel layer will be evaporated off the thin-layer chromatographic plate with just a few shots of the evaporating laser:
The same disadvantages will be found in an apparatus contructed following patent application GB 2 149 569 A. The lens shown in this arrangement will not be able to produce focal points less than a few 100 .mu.m. Lenses, also infrared-transparent lenses generally have surfaces of spherical shape that will result in strong optical aberrations at short focal lengths. The most important optical aberration in this case, called spherical aberration, will cause a blurring of the focal spot to a few 100 .mu.m. Silica gel, the most often used transport layer material for thin-layer chromatographic plates has an absorbtion length of a few 100 .mu.m. Thus, also in this case it is to be expected like in the method of Fanibanda et al. that each laser shot will evaporate such large amounts of material that just a few shots will evaporate the complete silica gel layer at the respective location on the plate. Likewise, each laser shot will produce such amounts of gas, that residual gas pressure will rise to unacceptable values.